fix mixer rect, textures popup, parallel abr import

src/brush.cpp

@@ -262,7 +262,7 @@ void Stroke::add_point(glm::vec3 pos, float pressure)
     float dist = m_keypoints.empty() ? m_step :
         m_keypoints.back().dist + glm::distance(m_keypoints.back().pos, pos);
     if (m_keypoints.empty())
-        m_prev_sample.origin = pos;
+        m_prev_sample = randomize_sample(pos, pressure, 0);
     else if (m_keypoints.back().pos == pos)
         return; // skip same point, leading to black samples (NaN values)
     Keypoint kp;